The present invention relates to a method of manufacturing a field-effect transistor, and, in particular, to a method of manufacturing a Schottky barrier gate type field-effect transistor (SB FET) formed on a GaAs semiconductor substrate.
GaAs MES FETs or GaAs SB FETs have excellent high-frequency properties and are generally used as microwave devices. Recently, MES FETs have also been used as elements for GaAs logic ICs. In this application, an improved manufacturing method of MES FETs is needed.
Various methods exist for manufacturing GaAs MES FETs. In a prevalent method, a channel region is formed in a GaAs substrate, a Schottky gate electrode of a refractory metal or a refractory metal compound is formed on the channel region, and n.sup.+ -type source drain regions are formed by self-alignment technology. In the formation of source and drain regions, n-type impurity is ion-implanted into the GaAs substrate by using the Schottky gate electrode as a mask, and the GaAs substrate is annealed without removing the Schottky gate electrode. The ion-implanted region in the GaAs substrate is activated by way of the annealing process. As examples of ion-implantation processes, a process of directly implanting impurity ions into an exposed semiconductor substrate, and a process of implanting impurity ions through an insulating film into a semiconductor substrate, are known. As examples of annealing processes for the source and drain regions, a cap annealing process and a capless annealing process are known. In the cap annealing method, the GaAs substrate is covered with an insulative protection film of, for example, PSG, AlN, SiO.sub.2, or SiN. In the capless annealing method, the GaAs substrate is exposed to an atmosphere of arsine gas (ASH.sub.3).
The above conventional methods have the following drawbacks:
(i) When impurity ions are implanted directly into a semiconductor substrate, a channeling phenomenon occurs. This prevents impurity atoms from distributing themselves within a semiconductor substrate as desired. The crystallinity of the semiconductor substrate is degraded in the ion-implantation process. The damaged crystal portion is susceptible to oxidation, contamination, and etching, compared to the other portion. If the surface of the semiconductor substrate continues to remain exposed even after the ion implantation and is subjected to various physical and chemical treatments, that may lead to unsatisfactory Schottky properties and non-uniform channel regions.
(ii) When impurity ions are implanted through an insulating film into a semiconductor substrate, this insulating film serves to protect the surface of the semiconductor substrate in an activation annealing process after the ion implantation. The insulating film must be removed from the surface of the semiconductor substrate before a Schottky gate electrode is formed by deposition. Therefore, satisfactory Schottky properties and uniform channel regions cannot readily be obtained in the produced MES FETs.
(iii) When the cap or capless annealing process is performed in order to form the n.sup.+ -type source and drain regions, stress-differences appear in the semiconductor substrate due to the thermal expansion coefficient differences between the gate electrode metal and the other material, for example, of the protection film. The stress differences enhance the lateral diffusion of the impurites which are ion-implanted, corresponding to the source and drain regions to be formed. As a result, a short-channel effect occurs and degrades the element characteristics.